Evaluate removing subPath option from @harperdb/nextjs in favor of Next.js basePath

## Background

The `subPath` option in `@harperdb/nextjs` may be redundant now that Next.js's own `basePath` config achieves the same routing behavior. Setting `basePath: 'next-app'` in `next.config.js` causes Next.js to serve from `/next-app` without any Harper-side `subPath` configuration.

Using `basePath` also benefits from Next.js's native image and link handling for that prefix.

## Decision needed

- Remove `subPath` option from the extension (breaking change, document migration)
- Or keep it as an alias/wrapper around `basePath`

Status: In Progress in Jira.

🤖 Filed by Claude on behalf of Kris.